The production of acetic acid by carbonylation of methanol and/or a reactive derivative thereof is a known process, having been reviewed, for example, by Howard et al in Catalysis Today, 1993, 18, 325-254. Typical catalysts employed in homogeneously catalysed carbonylation processes are rhodium, iridium, or a combination of rhodium and iridium as described, for example, in EP-A-0 161 874, EP-A-0 849 248 and EP-A-1 123 265 respectively. Such catalyst metals are often used in combination with one or more promoter metals, such as rhenium (EP-A-0 728 726), or ruthenium and/or osmium (EP-A-0 643 034 & EP-A-0 728 727).
It is known that catalysts and catalyst promoter metals are typically present in methanol carbonylation processes as a mixture of catalytic species, some of which species are less catalytically or promotionally active than other species, for example, as described in D. Forster, J. Chem. Soc., Dalton Trans., 1979, 1639. It is further known that under certain circumstances the less active species can be more prone to precipitation. For example, U.S. Pat. No. 6,103,934 describes a rhodium catalysed methanol carbonylation process in which rhodium is present as a mixture of the active form Rh(CO)2I2− and the inactive form Rh(CO)2I4−, the inactive form being more prone to precipitation than the active form.
WO 03/106396 describes how the use of low concentrations of specified iodides can reduce precipitation in iridium-catalysed carbonylation processes. Further, WO 03/097567 describes how precipitation in a ruthenium-promoted iridium catalysed carbonylation process can be reduced by maintaining a defined amount of carbon monoxide in the acetic acid recovery stage of the process. However, the amount of iodide added to the process described in WO 03/106396 is dependent on the total amount of iridium catalyst present in the carbonylation process; and the defined amount of carbon monoxide maintained in the process described in WO 03/097567 is dependent on the total amount of ruthenium promoter present in the process. Neither WO 03/106396 nor WO 03/097567 describes the specific iridium or ruthenium species which can result in the formation of precipitates in the carbonylation process. Thus, the processes of WO 03/106396 and WO 03/097567 may result in more iodide, or more carbon monoxide being employed than is actually required to reduce precipitates, i.e. resources may be wasted.
Infrared spectroscopy has hitherto been applied to carbonylation processes, for example, in analysing the concentrations of components of a liquid reaction composition, and adjusting the concentrations in response thereto, as described in U.S. Pat. No. 6,552,221 and U.S. Pat. No. 6,103,934. However, U.S. Pat. No. 6,552,221 makes no mention of employing infrared to measure the concentrations of catalytic species present in the liquid reaction composition; and U.S. Pat. No. 6,103,934 requires that the concentrations of a variety of components of the reaction composition are determined, in particular, the concentrations of at least methyl iodide, water and the active catalytic species present in the reaction composition must be determined.